One day as I was playing on Grand Theft Auto IV, I hit a key and noticed my ELS was flashing extremly fast. I replicated my keystrokes and found If you hit the correct keys at the correct time, you ELS will flash exceptionally faster than it should.

Here is the glitch and how to use it:

Step 1: Make sure the ELS is off

Step 2: Turn on automatic mode (ALT+T default)

Step 3: Turn on the Siren

Step 4: Turn off the siren 1 second later, then (not at that same time, but immediately after) turn on ELS

NOTE:

If these steps are done correctly your ELS should start flashing fast. You have to perform this glitch every time the lights are turned off if you want them faster.
And you may have to do this a couple time for it to work.